Working Outside the Cell

Many signaling molecules interact with cell surface receptors to mobilize calcium from intracellular stores, eliciting transient changes in the concentration of intracellular calcium ([Ca2+]i), which acts as a second messenger. Mobilized cytoplasmic Ca2+ extruded through plasma membrane Ca2+ ATPases can increase local extracellular Ca2+ concentration [Ca2+]o, whereas store-operated Ca2+ entry can lead to a localized decrease in [Ca2+]o. Indeed, in gastric epithelial cells, the cholinergic agonist carbachol elicits a simultaneous increase in lumenal [Ca2+]o and decrease in serosal [Ca2+]o, because of the polarized distribution of Ca2+-handling molecules. Using a proteolytic assay to measure pepsinogen secretion, and ion-sensitive microelectrodes to measure pH and [Ca2+], Caroppo et al. investigated the effects of changing [Ca2+]o on alkaline secretion and pepsinogen secretion in isolated frog gastric mucosa. Simultaneous changes in lumenal and serosal [Ca2+]o comparable to those elicited by carbachol stimulated comparable secretion of pepsinogen and changes in gastric gland lumenal pH. Increasing only lumenal [Ca2+]o had a smaller effect on secretion, whereas decreasing serosal [Ca2+]o alone was ineffective. Pharmacological analysis implicated a previously identified extracellular Ca2+ receptor (CaR) in the response; moreover, citrate, added to buffer lumenal [Ca2+]o, prevented both the carbachol-dependent increase in lumenal [Ca2+]o and alkaline and pepsinogen secretion. In contrast, stimulating CaR pharmacologically or increasing lumenal [Ca2+]o did not affect [Ca2+]i, as measured either with intracellular microelectrodes or indirectly through activation of a Ca2+-sensitive K+ current. Thus, the authors propose that, in this system, changes in [Ca2+]o after Ca2+ mobilization from intracellular stores can act as a third messenger to stimulate secretion.